Carbon-carbon composite particles, their preparation and use therefore as negative electrode for li-ion batteries

ABSTRACT

A composite of particles comprising a high crystallinity carbon and a low crystallinity carbon, wherein the low crystallinity carbon exhibits an average lattice constant d=(002) of 0.350 nm or more and a crystallite size L=(002) in the diffraction of C axis of 25 nm or less, as characterized by wide-angle X ray diffraction measurements, the high crystallinity carbon exhibits an average lattice constant d=(002) of 0.338 nm or less and a crystallinity size L=(002) in the diffraction of C axis of 40 nm or more, as characterized by wide-angle X-ray diffraction measurements, the high crystallinity carbon having at least 50% of its external surface embedded within or surrounded by a matrix of low crystallinity carbon.

TECHNICAL FIELD

The present invention relates to composite carbon-carbon particlesobtained from a low crystallinity carbon and of a high crystallinitycarbon.

The present invention also relates to a process for preparing thecomposite carbon-carbon particles of the invention by dispersing andmixing together low crystallinity carbon particles with highcrystallinity carbon particles.

The present invention also relates to a process for preparing thecomposite carbon-carbon particles of the invention by mixing carbonparticles with a polymeric substance and by heating the mixture therebyobtained until carbonization of the polymeric substance on the surfaceof the carbon particles

A further aspect of the present invention is the use of the compositeparticles inter alia in the foundry industry, in sports equipment, inthe automobile and in aeronautic industries and the use of thoseparticles as constituents of electrode material in electrochemicalsystems.

PRIOR ART

Li-ion batteries are now considered one of the best existing powersources for portable electronics such as cell-phones, camcorders, laptopcomputer and power tools. A Li-ion cell typically consists of acarbon-based negative electrode (NE), a porous polymer membraneseparator (polypropylene and/or polyethylene) and a lithium transitionmetal oxide (LiMO₂, M=Co, Ni, or Mn) based positive electrode asdescribed in Nishi in Advances in Lithium-ion batteries edited by W.Schalkwijik Cluwer Academic/plenum publishers, 2002, page 233,electrodes are made by casting slurries of active materials,polymer-based binder (i.e. polyvinylidene difluoride, PVDF) and smallamounts of high surface area carbon onto metal foil current collectors.Mixtures of Li-salts and organic solvents provide an electrolyte mediumfor Li-ions to shuttle between the PE and NE. During charge, Li-ionsdeintercalate from the PE and intercalate into the NE, while the reversetakes place during discharge as mentioned in A. Webber and G. Blomgrenin Advances in Lithium-ion batteries, edited by W. Schalkwijik CluwerAcademic/plenum publishers, 2002, on page 185.

The evolving products require a Li-ion cell with a longer cycle-life,higher energy and charge/discharge rate capabilities. Long cycle-lifee.g. is critical for the Li-ion battery to last the lifetime of the hostdevice (such as: embedded electronics and medical prosthesis); and highcapacity and rate capability are needed for the EV, aerospace andmilitary applications.

For the development of Li-ions of such unique properties, batterymanufacturers and research groups have been investigating possibleapplications of new and/or modified PE and NE materials.

These included utilization of In/Si-based intermetallic alloys,metal-carbon and carbon-carbon composites as NE-materials, and mixedmetal-oxides as PE-material, as disclosed in R. Huggins in Handbook ofbattery materials edited by J. Besenhard Wiley-veh, 1999, page 359.

However, despite their high capacity, available NE-materials containingcarbon-carbon composite, present the drawback of limited cyclic life,when available NE-materials containing In/Si-based intermetallic alloysdespite their capacity present the drawback to be used commerciallithium-ion batteries.

Electrodes are made by casting slurries of active materials,polymer-based binder (i.e. polyvinylidene difluoride, PVDF) and smallamounts of high surface area carbon onto metal foil current collectors.Mixtures of Li-salts and organic solvents provide an electrolyte mediumfor Li-ions to shuttle between the PE and NE. During charge, Li-ionsdeintercalate from the PE and intercalate into the NE, while the reversetakes place during discharge.

The evolving products are demanding for Li-ion cells with longercycle-life, higher energy and charge/discharge rate capabilities. Longcycle-life e.g. is critical if the Li-ion battery should last thelifetime of the host device (such as: embedded electronics and medicalprosthesis); and high capacity and rate capability are needed for theEV, aerospace and military applications.

For the development of Li-ions of such unique properties, batterymanufactures and research groups have investigating possible applicationof new and/or modified PE and NE materials, including the utilization ofIn/Si-based inter-metallic alloys, metal-carbon and carbon-carboncomposites as NE-materials, and mixed metal-oxides as PE-material.

However, despite their high capacity, available NE-materials containingcarbon-carbon composite present the drawback of cyclic life, whenavailable NE-materials containing In/Si-based intermetallic alloysdespite their capacity present the drawback to be used commerciallithium-ion batteries.

There was therefore a need for a positive electrode and/or negativeelectrode material free of the drawbacks usually associated with thecorresponding prior art known materials and presenting inter alia atleast one of the following properties: a long cycle life, a highcapacity, a low self discharge, a good compatibility with a low volumeexpansion and with a low reactivity required for the safety ofbatteries.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph that represents the thermal profile of the heattreatment of a (Carbon#2: polymerized PF-matrix) prepared according to aprocess of the invention as described thereafter under “samplepreparation”.

FIGS. 2 a to 2 d are schematic pictorial model representations of thesteps occurring during processing of the proposed carbon-carboncomposite material for NE in Li-ion cells, in the case wherein carbon#1progressively covers carbon#2 particles with a network of carbon#1.

FIG. 3 A is the SEM image of the carbon-carbon material according tosample 1 hereinafter defined in Table I, carbon#1 is derived fromcarbonization of PF, which cover partially the surface of carbon#2(graphite).

FIG. 3B is the same SEM image as in FIG. 3A, but at a larger scale.

FIG. 4A is the SEM image of the carbon-carbon material according tosample 2 defined hereinafter in Table I, the carbon#1 is derived fromcarbonization of PF and covers almost totally the surface of carbon#2(graphite); the measured particle size of carbon#1 is from 39 nm to 500nm.

FIG. 4B is the same SEM image as in FIG. 4A, at a larger scale.

FIG. 5A is the SEM image of the carbon-carbon material according tosample 3 as defined hereinafter in Table I. The carbon#1 is derived fromcarbonization of PF, which covers partially the surface of carbon#2(graphite), but still some graphite particles are not covered at all.The particle size of carbon#1 is from 20 nm to 500 nm.

FIG. 5B is the same image as in FIG. 5A but at a larger scale.

FIG. 6A is an SEM image of the carbon-carbon material according tosample 4 as defined hereinafter in Table I. The carbon#1 is derived fromcarbonization of PF, which covers almost totally the surface of carbon#2(graphite). The carbon#1 is more compact and it's particle size is from20 nm to 500 nm.

FIG. 6B is the same image as in FIG. 6A but at a larger scale.

FIG. 7 shows Transmission Electron Micrograph a carbon-carbon compositeaccording to the invention, that has been prepared according to samples3 and 4 and beat treated to 2,500° C.

DESCRIPTION OF THE INVENTION

A first object of the present invention is constituted by compositeparticles of carbon, thereafter named Carbon#1-Carbon#2 compositeparticles, wherein Carbon#1 means a low crystallinity carbon andCarbon#2 means a high crystallinity carbon.

In the framework of the present invention, the expression Carbon#1 meanscarbon particles, having a low crystallinity characterized by wide-angleX ray diffraction measurements, i.e by an average lattice constantd=(002) of 0.350 nm or more and a crystallite size L=(002) in thedirection of C axis of 25 nm or less. Such low crystallinity carbon ismore extensively described in Carbon electrochemical and physicochemicalproperties, John Wiley, New York, 1988.

According to a preferred embodiment of the invention low crystallinitycarbons are selected in the group constituted by hard carbon, glassycarbons, polymer derived carbons and petroleum cokes.

In the framework of the present invention, the expression Carbon#2 meansa high crystallinity carbon characterized by wide-angle X raydiffraction measurements, i.e by an average lattice constant d=(002) of0.338 nm or less and a crystallinity size L=(002) in the direction of Caxis of 40 nm or more. Such high crystallinity carbon is moreextensively defined in Carbon electrochemical and physicochemicalproperties, John Wiley, New York, 1988. According to a preferredembodiment of the invention such high crystallinity carbon is selectedin the group constituted by graphite, preferably natural graphites, kishgraphite, pyrolytic graphite, gas-growth graphite or any artificialgraphite.

A preferred family of Carbon#1-Carbon#2 composite particles of theinvention is constituted by those composite particles having at leastone of the following physical properties:

-   -   a package density, according to the tap density method        associated to the apparatus commercialized under the name Logan        Instrument Corp. Model Tap-2, that is >0.5 g/cc.    -   a particle size, measured according to the SEM method associated        with apparatus Microtac model X100 Particle Analyser, ranges        from 0.5 to 100 micrometers; and    -   a specific surface area, measured according to the BET method,        ranging from 1 to 50 m²/g.

As shown inter alia in FIG. 2, the composite particles of the inventionmay be described as particles of Carbon#2 embedded or surrounded by amatrix (or a network) of Carbon#1.

Therefore, particles of a high crystallinity carbon (preferably agraphitic carbon) are embedded within or surrounded by the matrix of asecond carbon that has lower degree of crystallinity (graphitization),also known as hard carbon.

A second object of the present invention is constituted by the use ofthe composite particles according to the first object of the presentinvention as a constituent of an electrode material, preferably as aconstituent of a N-E (negative electrode) material in electrochemicalsystems, in the foundry industry, in the car and in aeronauticindustries, in sports equipment.

A third object of the present invention is constituted by negativeelectrodes comprising between 2 to 98% and preferably by thosecomprising at least 90 weight per cent of a composite particles of theinvention, the remaining being preferably constituted by at least onebinder. The binder is preferably of the PVDF type.

A fourth object of the present invention is constituted by a batterysystem comprising at least one electrode containing carbon-carboncomposite particles according the invention. Preferably, in the batterysystem of the invention, the electrode containing composite particles isa negative electrode.

According to an another embodiment, the battery system is of theN-E/electrolyte/PE type.

According to a further preferred embodiment of the invention the batterysystem consists, of wined/stacked layers or of winded/stacked layers ofelectrodes of electrodes, at least one of said electrodes comprisingcarbon-carbon composite particles according to the invention.

A preferred embodiment is constituted by Li-ion battery comprising anegative electrode, a positive electrode and a porous polymer membraneseparator for example of the Celguard type, wherein at least one of saidelectrodes is a negative electrode according to the invention.

A fifth object of the present invention is constituted by a process forpreparing the composite material particles according to the first objectof the invention. This process preferably comprises the steps ofdispersing and mixing preferably by using ball milling, at least oneCarbon#1 powder, preferably in the form of a slurry of a polymericsubstance that results in carbon particles by heat treatment. Thepolymeric substances preferably a polymer with a high carbon content, ispreferably dispersed in an organic solvent in a slurry that can easilystick on the surface of the carbon particles to be covered. Thepolymeric material is preferably dispersed in a liquid Phenolic Resin(PF). Appropriate phenolic resins are for example those commonly used inthe foundry industry. Phenolic resins are the most widely used resinbinders in the foundry industry. They are produced by polycondensationof phenols with formaldehyde. The three types of resins presented in thefollowing Table differ in catalyst and mole ratio of reactants used intheir preparation. Furthermore, they have different molecular structuresand reactivities and require different curing agents.

There are two further categories of epoxy resins that may besuccessfully used, namely the glycidyl epoxy, and non-glycidyl epoxyresins. The glycidyl epoxies are further classified as glycidyl-ether,glycidyl-ester and glycidyl-amine.

The diglycidyl ether of bisphenol-A (DGEBA) is a typical commercialepoxy resin and is synthesised by reacting bisphenol-A withepichlorobydrin in presence of a basic catalyst.

The Novolac Epoxy Resins are glycidyl ethers of phlenolic novolacresins.

The mixing process is advantageously continued until complete dispersionof Carbon#2 in the PF and preferably until vaporization of 40 to 60 wt %of the containing solvent (preferably water, or organic solvent such asan alcohol) from PF. Among preferred solvent for preparing the PFdispersions are water and organic solvents such as alcohols.

The heat treatment is preferably carried out at a temperature rangingfrom 400 to 2,800° Celsius, and more preferably at a temperature rangingfrom 1,000 to 2,500° C.

A sixth object of the present invention is constituted by a process forpreparing a negative electrode. This process comprises the steps of:

-   -   a) dispersing and mixing, preferably by using ball milling, at        least one Carbon#1 powder, in preferably a liquid Phenolic Resin        (PF), the mixing process being preferably continued until        complete dispersion of carbon#2 in the PF and preferably until        vaporization of 40 to 60 wt % of the containing solvent        (preferably water, or organic solvent such as an alcohol) from        the PF;    -   b) pouring the mixture obtained in the preceding step on a        support, preferably on a flat Al-plate, on a Cu-plate, on an        alu-Exmet or on a cupfer Exmet, then heated preferably to        150-175° C., more preferably to 160-170° C. and thermally        soaked, for 1 to 5 hours, preferably for about 2.0 hours, the        heating rate varying preferably from 3-8° C/min depending on the        thickness of the sample;    -   c) after the preceding heating step, preferably converting the        sample into solid sheets from which the support (AL-plate was        separated); and    -   d) treating the Carbon#2: polymerized PF-matrix obtained in the        preceding step at a temperature ranging from 600 to 2.500° C.,        preferably at a temperature ranging from 600 to 1.000° C. using        30-50° C/min heating rate following thermal profile shown in        FIG. 1. Then the carbon#1 is produced by carbonization of        PF-matrix.

Any polymeric material, particularly any polymeric material with a highcarbon content and more preferably any PF transforms to graphite up tobeating through the following process:

-   -   1. PF Polymerizes to a rubbery gel on heating to 85° C. (gelling        process);    -   2. on heating to 120° C., the rubbery gel cures to form a hard        cross-linked polymer by condensation reaction which produces        water;    -   3. on heating to 225° C., the hard cross-linked polymer obtained        in step 2 forms yellowish transparent material with lower        density that its previous stages, this coincides with loss of        more water and material having high porosity, at this stage        neighboring carbon chains merge and start forming 3D-carbon        matrixes;    -   4. on heating to from 225° C. to 500° C., the material obtained        in the preceding step becomes free of water and yet contains        appreciable amounts of hydrogen, this step is know as        pre-carbonization carbonization step which was critical in        controlling the porosity of the host material. Slower heating        leads to smaller pore sizes;    -   5. on heating to 1000-1250° C., material volume shrinks, its        electrical conductivity increases by many orders of magnitudes;    -   6. at temperatures above 1200° C., the material being depleted        of hydrogen; and    -   7. further cross-linking and carbon chains starts growing in 2D        & 3D directions, this means that carbon#2 sticks on the surface        of the carbon#2 particles and carbon#1 squeezes into itself with        the temperature is increased to 2.500° C.,

In the following examples, the PF material used was supplied in the year2001 by Georgia Pacific Co., Lawrenceville, Ga., in the United States ofAmerica under the reference Products # PF211.

Here, we are proposing the application of a new Carbon#1-Carbon#2composite as N-E material for battery systems consisted ofwinded/stacked layers of electrodes with potential differences betweenthem being dependant on their electrochemical properties. The otherdisclosing element of this proposal is the processing of thecarbon-carbon composites noted above. The proposed material consists ofcarbon-carbon composite of 10-100 μm particles.

This consisted of a highly graphitic carbon (e.g. graphite) embeddedwithin or surrounded by the matrix of a second carbon that has a lowdegree of graphitization known as hard carbons).

The low crystallinity carbon constituting of Carbon#1 has a sloppyvoltage and a high medium voltage about 500 mV vs Li⁺/Li. This highsloppy voltage will be affected on the decreasing of the energy densityof the batteries. However this is compatible with PC based electrolyte.This type of electrolyte is suitable for low temperature applications.

The high crystallinity carbon constituting of Carbon#2 has a flatvoltage and low medium voltage about 100 mV vs Li⁺/Li. This low flatvoltage will be affected on the increasing of the energy density of thebatteries. However it is not compatible with PC based electrolyte.

EXAMPLES

The following examples are given for illustrative purpose only and maynot be construed as consituting a limitation of the present invention.

Sample Preparation:

The following steps were used to prepare the proposed material:

-   -   1—Carbon#2 powder was dispersed in Liquid Phenolic Resin (PF)        and the mixed using mixture ball milling. The mixing process was        continued until complete dispersion of Carbon#2 in the PF and        vaporization of 40- 60wt % of the containing alcohol from PF,        both were insured.    -   2—item-1 mixture was wed poured on flat Al-plate, then heated to        150-175° C. and thermally soaked for 2.0 hours. The heating rate        could vary from 3 -8° C/min depending on the thickness of the        sample, in our case heating of 5° C/min was used. After this        heating process, the sample converts into solid sheets from        which the AL-plate was separated.    -   3—item-3 Carbon#2: polymerized PF-matrix) heat treated to        600° C. and then to 1000 or 2500° C. using 30-50° C/min heating        rate following thermal profile shown in FIG. 1.

The PF matrix polymerizes and pre-carbonizes up to 450-475° C. Duringthis step, PF generates water, which vaporizes while heating. This leadsto increasing density and decreasing volume. Between 500-600° C. PFbegins the early stages of carbonization steps where the carbongraphite-sheets start buckling-up and building three-dimensionalmatrixes of randomly oriented short carbon layers with porous structure.PF porosity could depend on heat rate and thermal-soaking time in thistemperature range. For longer heating time between 475-600° C. the PFpores size to become smaller. Above 600° C., the graphitization stepstarts and the graphite layer cross-link further. This process tends tosqueeze the carbon#1 particles closer together and providescarbon-carbon composites with the low graphitized carbon matrix pressingover the highly graphitized carbon particles.

FIG. 7 shows Transmission Electron Micrograph of the carbon-carboncomposite heat treated to 2.500° C. and as prepared according to sample3 and 4.

The following Table 1 lists the data of carbon-carbon composite samplesprepared for proof of the concept.

TABLE I Bulic Sample Pore Area Ave. Density Skeletal No. Description(m²/g) Dia. (μ) (g/cm3) Density 1 100 g PF 9.184 ± 0.02 0.482 0.5841.267 50 g-SPG-44 1000° C. 2 100 g PF 13.193 ± 1    0.243 0.605 1.162 40g-SFG-44/5% LiNO3 1000° C. 3 100 g PF 3.198 ± 0.05 0.123 0.988 1.095 70g-SFG-44 2500° C. 4 100 g PF 6.550 ± 0.12 0.437 0.532 1.413 70 g-SFG-152500° C.

Carbon#1 is a carbon derived by heat treatment from 1.000 to 2.500Celcius preferably at 1.000 degrees Celcius) from phenolic resin.

Carbon#2 is an artificial graphite—commercialized under the name SFG 44(particles having a size of 44 micrometers) and under the name SFG 15(particles having a size of 15 micrometers by the Company Timcal(anciently Lonza in Swiss).

Example 1

In this example the carbon-carbon composite was made by mixing 100 g PFwith 50 g of SFG-44 (artificial graphite) commercialized by Timcal(Swiss) and heated at 1.000° C. for 2 hours in argon atmosphere. The SEMimage of the carbon-carbon material of the sample 1 is shown in FIGS.3A-B. The carbon#1 is derived from carbonization of PF, which coverpartially the surface of carbon#2 (graphite). The electrochemicalperformance was obtained by using 1M LiClO₄ in EC-DMC as electrolyte.Three electrodes cell was used with lithium metal as counter electrodeand reference. The reversible capacity of sample #1 is 218 mAh/g at C/12rate (charge and discharge in 12 hours). The coulombic efficiency of thefirst cycle was 85%.

Example 2

In this example the carbon-carbon composite was made by mixing 100 g PFwith 40 g-SFG-44/5%LiNO3 and heated at 1000° C. for 2 hours in argonatmosphere. The SEM image of the carbon-carbon material of the sample 2is shown in FIGS. 4A-4B. The carbon#1 is derived from carbonization ofPF, which cover almost totally of the surface of carbon#2 (graphite).The particle size of carbon#1 is from 39 nm to 500 nm. Theelectrochemical performance was obtained by using 1M LiClO₄ in EC-DMC aselectrolyte. Three electrodes cell was used with lithium metal ascounter electrode and reference. The reversible capacity of sample #2 is259 mAh/g at C/12 rate (charge and discharge in 12h). The coulombicefficiency of the first cycle was 82%.

Example 3

In this example the carbon-carbon composite was made by mixing 100 g PFwith 70 g-SFG-15 and heated at 2500° C. for 1 minute in argonatmosphere. The SEM image of the carbon-carbon material of the sample 3is shown in FIGS. 5A-B. The carbon#1 is derived from carbonization ofPF, which cover partially the surface of carbon#2 (graphite), but stillsome graphite particle not covered at all. The particle size of carbon#1is from 20 nm to 500 nm

Example 4

In this example the carbon-carbon composite was made by mixing 100 g PFwith 70 g-SFG-15 and heated at 2500° C. for 2 hours in argon atmosphere.The SEM image of the carbon-carbon material of the sample 3 is shown inFIGS. 6A-6B. The carbon#1 is derived from carbonization of PF, whichcover almost totally the surface of carbon#1 (graphite). The carbon#1 ismore compact, it's particle size is from 20 nm to 500 nm.

The electrochemical performance was obtained by using 1M LiClO₄ inEC-DMC as electrolyte. A three electrodes cell was used with lithiummetal as counter electrode and reference. The reversible capacity ofsample #4 is 280 mAh/g at C/12 rate (charge and discharge in 12 h). Thecoulombic efficiency of the first cycle was 56%

Among the improved properties of the new carbon-carbon material of thepresent invention over the known materials and particularly over theknown NE materials are inter alia:

-   -   the compatibility with propylene carbonate (PC);    -   the low volume expansion;    -   the high electrode density; and    -   the low reactivity (safety).

It is to be understood that, while the foregoing invention has beendescribed in detail by way of illustration and example, numerousmodifications, substitutions, and alterations are possible withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

1. A composite of particles comprising a high crystallinity carbon and alow crystallinity carbon, the composite being obtained by a processcomprising: dispersing and mixing the high crystallinity carbon in apolymeric substance form a mix; and subjecting the mix to a heattreatment at a temperature of at least 1200° C. to form a composite withhigh crystallinity carbon embedded within or surrounded by a matrix ofpolymer derived low crystallinity carbon; wherein: the low crystallinitycarbon exhibits an average lattice constant d=(002) of 0.350 nm or moreand a crystallite size L=(002) in the diffraction of C axis of 25 nm orless, as characterized by wide-angle X ray diffraction measurements; thehigh crystallinity carbon exhibits an average lattice constant d=(002)of 0.338 nm or less and a crystallinity size L=(002) in the diffractionof C axis of 40 nm or more, as characterized by wide-angle X-raydiffraction measurements; the high crystallinity carbon having at least50% of its external surface embedded within or surrounded by a matrix oflow crystallinity carbon; and the high crystallinity carbon formsparticles having a particle size ranging from 1 to 50 micrometers, thehigh crystallinity carbon particles being at least partially covered byparticles of low crystallinity carbon having a particle size rangingfrom 10 to 500 nm, the low crystallinity carbon particles being attachedto the surface of the high crystallinity carbon particles.
 2. Acomposite of particles according to claim 1, wherein the highcrystallinity carbon is selected in the group consisting of graphite,kish graphite, pyrolytic graphite, gas-growth graphite and anyartificial graphite.
 3. A composite of particles according to claim 1,wherein the high crystallinity carbon is a natural graphite.
 4. Acomposite of particles according claim 1, wherein the package density ofsaid particles, measured according to the tap density method associatedto the apparatus commercialized under the name Logan Instrument Corp.Model Tap-2, is superior or equal to 0.5 g/cc.
 5. A composite ofparticles according to claim 1, wherein the particle size of saidparticles, measured according to the SEM method, associated to theapparatus Microtrac model X100 Particle Analyser, ranges from 0.5 to 100micrometers.
 6. A composite of particles according to claim 1, havingaccording to the BET method, a specific surface area ranging from 1 to50 m²/g.
 7. A composite of particles according to claim 1, wherein atleast 70% of the external surface of the high crystallinity carbon isembedded or surrounded by a matrix of low crystallinity carbon.
 8. Acomposite of particles according to claim 1, having a size measuredaccording to the SEM (Scanning Electron Microscopy) method ranging from1 to 50 micrometers.
 9. A composite of particles according to claim 1,having a size measured according to the SEM (Scanning ElectronMicroscopy) method ranging from 12 to 20 micrometers.